1. Field of the Invention
The present invention relates to an air/fuel ratio controller for an internal combustion engine which makes use of a hydrocarbon-adsorbent exhaust purification catalyst.
2. Related Background Art
Nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), and other such substances in the exhaust gas of an internal combustion engine are purified with a three-way catalyst installed along the exhaust path. Four-way catalysts, which purifies particulate matters (PM) in addition to the above-mentioned substances, are also used with diesel engines. However, these catalysts only exhibit their purification performance at a specific activation temperature. Also, since there is a tendency for a large quantity of hydrocarbons to be emitted immediately after cold start-up, a hydrocarbon adsorbent having the property of adsorbing hydrocarbons is sometimes disposed along the exhaust path.
Hydrocarbon adsorbent adsorb hydrocarbons at low temperature, and the adsorbed hydrocarbons are then desorbed once the adsorbent reaches a certain temperature. In the desorption of hydrocarbons, the catalyst reaches its activation temperature and is able to purify the desorbed hydrocarbons. This process makes it possible to reduce the hydrocarbons released into the atmosphere after cold start-up. Hydrocarbon-adsorbent exhaust purification catalysts comprising a hydrocarbon adsorbent supported on a catalyst have also been put to practical use. Internal combustion engines featuring such hydrocarbon-adsorbent exhaust purification catalysts are known from their disclosure in Japanese Laid-Open Patent Application H11-82111 and elsewhere.
A hydrocarbon-adsorbent exhaust purification catalyst combines the properties of the above-mentioned hydrocarbon adsorbent and the properties of an exhaust purification catalyst. Even a hydrocarbon-adsorbent exhaust purification catalyst, though, does not reach its activation temperature immediately after the cold start-up, so the large quantity of hydrocarbons released after the cold start-up cannot be adequately purified. Therefore, a hydrocarbon-adsorbent exhaust purification catalyst first adsorbs hydrocarbons immediately after cold start-up and desorbs the hydrocarbons as its own temperature rises. Then it uses its own exhaust purification function to oxidize and purify the desorbed hydrocarbons. When the hydrocarbons are desorbed, the hydrocarbon-adsorbent exhaust purification catalyst reaches its activation temperature.
The apparatus disclosed in the above-mentioned publication has a start-up catalyst (exhaust purification catalyst) disposed upstream along an exhaust passage so that it will be quickly heated up to its activation temperature by the hot exhaust gas, and a hydrocarbon-adsorbent exhaust purification catalyst disposed downstream from the start-up catalyst. While hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, feedback control (lean control for facilitating hydrocarbon oxidation) is performed on the basis of the output of an air/fuel ratio sensor disposed downstream from the hydrocarbon-adsorbent exhaust purification catalyst so that it will be easier to oxidize the desorbed hydrocarbons. When no hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, the system switches over to feedback control on the basis of an air/fuel ratio sensor disposed upstream from the start-up catalyst, as in normal operation.
In the apparatus disclosed in the above publication, since feedback control is performed on the basis of the air/fuel ratio sensor disposed further downstream from the downstream hydrocarbon-adsorbent exhaust purification catalyst, it takes some time for changes in the load of the internal combustion engine, external disturbances (caused by purge gas, etc.), and so forth to be reflected in the output of the air/fuel ratio sensor. Because the feedback control is based on the output of this downstream air/fuel ratio sensor, the effect of the oxygen occlusion actions of the start-up catalyst and the hydrocarbon-adsorbent exhaust purification catalyst, for example, results in it taking longer to return to the target air/fuel ratio, which tends to delay the control. As a result, exhaust purification efficiency may suffer, and there is the danger that controllability with respect to external disturbances and so on will be poor.
It is an object of the present invention to provide an air/fuel ratio controller for an internal combustion engine, with which the air/fuel ratio can be optimized even while adsorbed hydrocarbons are being desorbed from a hydrocarbon-adsorbent exhaust purification catalyst, and deterioration of exhaust purification performance can be minimized.
The air/fuel ratio controller for an internal combustion engine of the present invention comprises a upstream catalyst disposed upstream along an exhaust passage, a hydrocarbon-adsorbent exhaust purification catalyst that is disposed downstream from the upstream catalyst and has the function of adsorbing hydrocarbons at low temperatures and releasing the adsorbed hydrocarbons as the temperature rises, upstream air/fuel ratio detection means disposed upstream from the upstream catalyst, for detecting the exhaust air/fuel ratio of exhaust gas flowing into the catalyst, and downstream air/fuel ratio detection means disposed downstream from the hydrocarbon-adsorbent exhaust purification catalyst, for detecting the exhaust air/fuel ratio of exhaust gas flowing out of the hydrocarbon-adsorbent exhaust purification catalyst. The present invention also comprises air/fuel ratio main feedback control means for performing feedback control such that the exhaust air/fuel ratio detected by the upstream air/fuel ratio detection means is kept at a specific main feedback target air/fuel ratio, and air/fuel ratio sub-feedback control means for performing sub-feedback control such that the exhaust air/fuel ratio detected by the downstream air/fuel ratio detection means is kept at a specific sub-feedback target air/fuel ratio while the adsorbed hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst (xe2x80x9cduring hydrocarbon desorptionxe2x80x9d).
Accordingly, with the present invention, air/fuel ratio main feedback control is performed on the basis of the detection result from the upstream air/fuel ratio detection means. And, during hydrocarbon desorption, air/fuel ratio sub-feedback control is performed on the basis of the detection result from the downstream air/fuel ratio detection means on the downstream side of the hydrocarbon-adsorbent exhaust purification catalyst. xe2x80x9cSub-feedback controlxe2x80x9d refers to feedback control performed subordinately to main feedback control, and main feedback control takes precedence in overall control. The sub-feedback control adds fine corrections to the main feedback control so that the object of feedback control will stay at the target value. The object of main feedback control here is the exhaust air/fuel ratio of the exhaust gas flowing into the upstream catalyst, while the object of the sub-feedback control during hydrocarbon desorption is the exhaust air/fuel ratio of the exhaust gas flowing out of the hydrocarbon-adsorbent exhaust purification catalyst.
During hydrocarbon desorption, that is, in a state in which desorbed hydrocarbons and the hydrocarbons contained in the injected fuel must be oxidized (purified), sub-feedback control is performed on the basis of the exhaust air/fuel ratio of the exhaust gas flowing out of the hydrocarbon-adsorbent exhaust purification catalyst, so precise exhaust purification can be achieved. Furthermore, main feedback control based on the exhaust air/fuel ratio of the exhaust gas flowing into the upstream catalyst is also performed at this time.
And, even though the sub-feedback control may take some time to feedback, the overall control of the air/fuel ratio will be carried out precisely by main feedback control. So there will be no deterioration in exhaust purification performance due to a fluctuating air/fuel ratio or the like.
Furthermore, the present invention also comprises middle air/fuel ratio detection means disposed between the upstream catalyst and the hydrocarbon-adsorbent exhaust purification catalyst, for detecting the exhaust air/fuel ratio of exhaust gas flowing into the hydrocarbon-adsorbent exhaust purification catalyst. With the above-mentioned air/fuel ratio sub-feedback control means, after the adsorbed hydrocarbons have been desorbed from the hydrocarbon-adsorbent exhaust purification catalyst (xe2x80x9cafter hydrocarbon desorptionxe2x80x9d), sub-feedback control is performed so that the exhaust air/fuel ratio detected by the middle air/fuel ratio detection means is kept at a specific sub-feedback target air/fuel ratio. Specifically, the object of sub-feedback control after hydrocarbon desorption is the exhaust air/fuel ratio of the exhaust gas flowing into the hydrocarbon-adsorbent exhaust purification catalyst.
Consequently, after hydrocarbon desorption, sub-feedback control is performed on the basis of the exhaust air/fuel ratio of the exhaust gas flowing into the hydrocarbon-adsorbent exhaust purification catalyst as detected by the middle air/fuel ratio detection means. After hydrocarbon desorption, no hydrocarbons are desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, so exhaust purification cannot be effectively performed by performing sub-feedback control on the basis of the exhaust air/fuel ratio of the exhaust gas flowing into the hydrocarbon-adsorbent exhaust purification catalyst. Furthermore, main feedback control based on the exhaust air/fuel ratio of the exhaust gas flowing into the upstream catalyst is performed as discussed above, and even though the sub-feedback control Lakes some time, the overall control of the air/fuel ratio is more precise, so there is no deterioration of the exhaust purification performance due to a fluctuating air/fuel ratio or the like.
It is preferable here to vary the sub-feedback target air/fuel ratio of the middle air/fuel ratio detection means in effect after hydrocarbon desorption with respect to the sub-feedback target air/fuel ratio of the downstream air/fuel ratio detection means in effect during hydrocarbon desorption. Since the optimal sub-feedback target air/fuel ratio depends on whether hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, varying the ratio in this way allows air/fuel ratio sub-feedback control to be performed more precisely according to each situation.
In particular, it is preferable here if the sub-feedback target air/fuel ratio of the downstream air/fuel ratio detection means in effect during hydrocarbon desorption is set to be leaner than the sub-feedback target air/fuel ratio of the middle air/fuel ratio detection means in effect after hydrocarbon desorption. During hydrocarbon desorption, the hydrocarbons desorbed from the hydrocarbon-adsorbent exhaust purification catalyst have to be oxidized (purified) in addition to the hydrocarbons contained in the exhaust gas after combustion in the engine, but setting the target for the downstream exhaust air/fuel ratio of the hydrocarbon-adsorbent exhaust purification catalyst leaner allows the hydrocarbons being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst to be effectively oxidized as well, and minimizes deterioration of exhaust purification performance.
Also, the air/fuel ratio controller for an internal combustion engine of the present invention comprises a upstream catalyst, and a hydrocarbon-adsorbent exhaust purification catalyst that is disposed downstream from the upstream catalyst and has the function of adsorbing hydrocarbons at low temperatures and releasing the adsorbed hydrocarbons as the temperature rises. This is characterized in that during the desorption of hydrocarbon from the hydrocarbon-adsorbent exhaust purification catalyst, air/fuel ratio main feedback control is performed by an air/fuel ratio sensor upstream from the catalyst, and air/fuel ratio sub-feedback control, in which the target air/fuel ratio of the air/fuel ratio main feedback control is corrected, is performed such that the air/fuel ratio sensor output on the downstream of the hydrocarbon-adsorbent exhaust purification catalyst will be the target output.
With the present invention, feedback for controlling the amount of fuel injection is faster because air/fuel ratio main feedback control is performed by an air/fuel ratio sensor disposed upstream of the upstream catalyst. The exhaust purification performance is also enhanced because air/fuel ratio sub-feedback control, in which the target air/fuel ratio of the air/fuel ratio main feedback control is corrected, is performed such that the air/fuel ratio sensor output on the downstream of the hydrocarbon-adsorbent exhaust purification catalyst is kept at the target output. As a result, even if there is engine load fluctuation or external disturbance during the hydrocarbon desorption, retardation of the timing at which the fuel injection quantity is fed back with respect to air/fuel ratio fluctuation can be minimized and better exhaust purification performance obtained. The xe2x80x9cair/fuel ratio sensorxe2x80x9d here may be a so-called oxygen sensor whose output varies sharply depending on whether the exhaust air/fuel ratio is on lean or rich, or it maybe a so-called linear air/fuel ratio sensor that linearly monitors the exhaust air/fuel ratio from rich to lean.
After the desorption of hydrocarbons from the hydrocarbon-adsorbent exhaust purification catalyst, it is preferable here if air/fuel ratio sub-feedback control, in which the target air/fuel ratio of the air/fuel ratio main feedback control is corrected, is performed such that the air/fuel ratio sensor output on the downstream of the hydrocarbon-adsorbent exhaust purification catalyst is kept at the target output. When this is done, after hydrocarbon desorption from the hydrocarbon-adsorbent exhaust purification catalyst, no hydrocarbons are desorbed from the hydrocarbon-adsorbent exhaust purification catalyst, so exhaust purification can be effectively performed by performing air/fuel ratio sub-feedback control on the basis of the exhaust air/fuel ratio of the exhaust gas flowing into the hydrocarbon-adsorbent exhaust purification catalyst.
Furthermore, it is preferable here if the target output of the air/fuel ratio sensor during the desorption of hydrocarbons from the hydrocarbon-adsorbent exhaust purification catalyst is different from that after desorption. Doing this allows precise air/fuel ratio sub-feedback control to be performed according to whether hydrocarbons are being desorbed from the hydrocarbon-adsorbent exhaust purification catalyst. In particular, it is preferable here if the target output of the air/fuel ratio sensor during the desorption of hydrocarbons is set to be leaner than the target output after desorption. Doing this allows the hydrocarbons desorbed from the hydrocarbon-adsorbent exhaust purification catalyst to be effectively oxidized as well, and in particular allows the exhaust purification performance during hydrocarbon desorption to be improved.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.